Abstract
Tuberous sclerosis is a largely benign tumor syndrome derived from the acquisition of somatic lesions in genes encoding the tumor suppressor products, TSC1 or TSC2. Loss of function of the TSC1-TSC2 complex, which acts as a Rheb GAP, yields constitutive, unrestrained signaling from the cell growth machinery comprised of Rheb, mTOR, and S6K. We demonstrate herein that constitutive activation of the Rheb/mTOR/S6K cassette, whether by genetic deletion of TSC1 or TSC2 or by ectopic expression of Rheb, is sufficient to induce insulin resistance. This is the result of downregulation of the insulin receptor substrates, IRS1 and IRS2, which become limiting for signal transmission from the insulin receptor to PI3K. Downstream of PI3K, the survival kinase, Akt, is completely refractory to activation by IRS-dependent growth factor pathways such as insulin or IGF-I in TSC1- or TSC2-deficient cells but not to activation by IRS-independent pathways such as those utilized by PDGF. The antiapoptotic program induced by IGF-I but not PDGF is severely compromised in TSC2 null cells. Our results suggest that inappropriate activation of the Rheb/mTOR/S6K pathway imposes a negative feedback program to attenuate IRS-dependent processes such as cell survival.
Publication types
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Comparative Study
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Research Support, U.S. Gov't, P.H.S.
MeSH terms
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Blotting, Northern
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Cell Survival / physiology
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Cells, Cultured
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Down-Regulation*
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Flow Cytometry
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Humans
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Immunoblotting
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Immunoprecipitation
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Insulin Receptor Substrate Proteins
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Insulin Resistance / physiology*
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Insulin-Like Growth Factor I / metabolism
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Monomeric GTP-Binding Proteins / metabolism
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Neuropeptides / metabolism
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Phosphatidylinositol 3-Kinases / metabolism
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Phosphoproteins / metabolism*
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Protein Kinases / metabolism
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Proteins / metabolism
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Ras Homolog Enriched in Brain Protein
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Repressor Proteins / metabolism
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Ribosomal Protein S6 Kinases / metabolism
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Signal Transduction*
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TOR Serine-Threonine Kinases
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Tuberous Sclerosis / metabolism
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Tuberous Sclerosis / physiopathology*
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Tuberous Sclerosis Complex 1 Protein
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Tuberous Sclerosis Complex 2 Protein
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Tumor Suppressor Proteins
Substances
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IRS1 protein, human
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Insulin Receptor Substrate Proteins
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Neuropeptides
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Phosphoproteins
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Proteins
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RHEB protein, human
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Ras Homolog Enriched in Brain Protein
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Repressor Proteins
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TSC1 protein, human
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TSC2 protein, human
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Tuberous Sclerosis Complex 1 Protein
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Tuberous Sclerosis Complex 2 Protein
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Tumor Suppressor Proteins
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Insulin-Like Growth Factor I
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Protein Kinases
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MTOR protein, human
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Ribosomal Protein S6 Kinases
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TOR Serine-Threonine Kinases
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Monomeric GTP-Binding Proteins